The M2 Machine Gun (Browning .50 Caliber Machine Gun, Ma Deuce) is a heavy machine gun designed towards the end of World War I by John Browning. It was nicknamed Ma Deuce by US troops or simply called "fifty-cal" in reference to its caliber. The design has had many specific designations; the official designation for the infantry type is Browning Machine Gun, Cal. .50, M2, HB, Flexible.

The Browning .50 machine gun has been used extensively as a vehicle weapon and for aircraft armament by the United States from the 1920s to the present day. It was heavily used during World War II, the Korean War, the Vietnam War, as well as during operations in Iraq in the 1990s and 2000s. It is the primary heavy machine gun of NATO countries, and has been used by many other countries. It is still in use today. It was very similar in design to the smaller M1919 Browning machine gun.30-06 Springfield.

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Using a round originally designed by Winchester, the .50 BMG round was designed as a response to the German 13mm antitank rifles of World War I and employed in a redesigned and scaled-up M1917 Browning .30 cal machine gun. It was quickly adapted to the anti-aircraft role. It was also selected for the ground role and adopted by the U.S. as the Model 1921. The latter served during the 1920s as an anti-aircraft and anti-armor gun. In 1932, the design was updated and adopted as the M2, though carrying out the same functions. With the addition of a thicker walled barrel for better cooling (though counter-intuitive, a thicker barrel has a larger surface area, so provides better air-cooling) it became the M2HB (for Heavy Barrel). Due to the long procedure for changing the barrel, an improved system was developed called QCB (quick change barrel). A lightweight version, weighing 24 lb (11 kg) less—a mere 60 lb (27 kg)—was also developed.

The M2 is a scaled-up version of John Browning's M1917 .30 caliber machine gun (even using the same timing gauges), and fires the .50 BMG cartridge, which today is also used in high-powered sniper rifles and long range target rifles due to its excellent long range accuracy, external ballistics performance, incredible stopping power, and lethality. The M2 is an air-cooled, belt-fed, machine gun that fires from a closed bolt, operated on the long recoil principle. In this action, the bolt and barrel are initially locked together, and recoil upon firing. After a short distance, the bolt and barrel unlock, and the bolt continues to move rearwards relative to the barrel. This action opens the bolt, and pulls the belt of ammunition through the weapon, readying it to fire again, all at a cyclic rate of 450-550 rounds per minute. This is a rate of fire not generally achieved in use, as sustained fire at that rate will "shoot out" the barrel within a few thousand rounds, necessitating replacement. The M2 machine gun's sustained rate of fire is considered to be anything less than 40 rounds per minute.

The M2 has a maximum range of 7.4 kilometers (4.2 miles) when using the M2 ball ammunition, with a maximum effective range of 1.8 kilometers (1.2 miles) when fired from the M3 tripod. In its ground-portable, crew-served role, the gun itself weighs in at a hefty 84 pounds (38 kg), and the assembled M3 tripod another 44 pounds (20 kg). In this configuration, the V-shaped trigger is located at the very rear of the weapon, with a "spade handle" hand-grip on either side of it and the bolt release in the center. The "spade handles" are gripped and the trigger is depressed with one or both thumbs. When the bolt release is in the up position, the weapon is in single-shot mode. The bolt release must be pressed each time the weapon is fired to close the bolt and reload the weapon. The bolt release can be locked into the down position resulting in fully-automatic firing.

Because the M2 was designed with intent in many configurations, it can be adapted to feed in rounds from the left or right side of the weapon by exchanging the belt-holding pawls, the belt feed pawl, the front and rear cartridge stops and reversing the bolt switch. The conversion can be completed within a minute with no tools.

When firing blanks, a large blank-firing adapter (BFA) must be used, in order to keep the gas pressure high enough to allow the action to cycle. It is very distinctive, with attachment to the muzzle and three rods extending back to the base. The BFA can often be seen on M2s in peacetime operations.

When doubled it is used as an anti-aircraft gun in some ships, or on the ground. In these cases a pair of one left-handed and one right-handed feeds are used. In some cases four to six guns are mounted on the turret.

A primary armament in WWII-era U.S. aircraft such as the P-51 Mustang, and the Korean-era U.S. F-86 Sabre.

Defensive armament in WWII-era bombers like the B-17 Flying Fortress, and B-24 Liberators.

A long range sniper rifle, when attached with a scope. One well-known expert was US Marine sniper Carlos Hathcock during the Vietnam War. The success of the M2 in this role led to the development of actual sniper rifles based on the same .50 caliber round.

Can Be mounted on to vehicles like the HMMWV.

A water-cooled version of the larger M2 was used as an emplaced or vehicle-mounted anti-aircraft weapon on a sturdy pedestal mount.

Commonwealth use of the .50 was limited in the Second World War, despite it being standard armament on US-built/designed AFVs such as the M4 Sherman or M10 Wolverine that began to see use in British, Canadian, Australian and New Zealand units from 1942 on. Commonwealth tank crew commanders more often than not deleted the .50 altogether as being of limited use, given several factors. Primarily, the weapon was an anti-aircraft weapon, and Allied aerial superiority precluded its necessity. As well, in order to employ the weapon against ground targets, the commander had to exit the turret and expose himself to enemy fire. Commanders - especially in Italy - also found that the gun caught on low-hanging trees and vines and posed a danger to the crew commander's head and face.

The basic M2 was deployed in US service in a number of subvariants, all with separate complete designations as per the US Army system. The basic designation as mentioned in the introduction is Browning Machine Gun, Cal. .50, M2, with others as described below.

The development of the M1921 water-cooled machine gun which led to the M2, meant that the initial M2s were in fact water-cooled. These weapons were designated Browning Machine Gun, Cal. .50, M2, Water-Cooled, Flexible. There was no fixed water-cooled version.

Improved air-cooled heavy barrel versions came in three subtypes. The basic infantry model, Browning Machine Gun, Cal. .50, M2, HB, Flexible, a fixed developed for use on the M6 Heavy Tank designated Browning Machine Gun, Cal. .50, M2, HB, Fixed, and a "turret type" whereby "Flexible" M2s were modified slightly for use in tank turrets. The subvariant designation Browning Machine Gun, Cal. .50, M2, HB, Turret was only used for manufacturing, supply, and administration identification and separation from flexible M2s.

Specific aircraft versions were also developed, and these subvariants are discussed in the following paragraph along with the AN/M2.

The M2 machine gun was heavily used as a remote fired fixed weapon, primarily in aircraft, but also in other applications. For this a variant of the M2 was developed (sometimes seen under the designation AN/M2, but it is important to note that there were .30 and .50 caliber weapons with this designation), with the ability to fire from a solenoid trigger. For aircraft mounting some were also fitted with substantially lighter barrels, permitted by the cooling effect of air in the slip-stream. The official designation for this weapon was Browning Machine Gun, Aircraft, Cal. .50, M2 followed by either "Fixed" or Flexible" depending on whether the weapon was used as a fixed forward firing gun or for use by an airplane's crew, such as a waist gun position on a B-17.

The M3 was a more purpose built variant for remote firing use, that also featured a higher rate of fire.

The XM296/M296 is a further development of the M2/M3 machine gun for remote firing applications, and is currently used in armament systems pertaining to the OH-58 Kiowa Warrior helicopter. The M296 differs from previous remote firing variants primarily in the lack of bolt latch allowing for single shots.

The GAU-10/A (NSN or National Stock Number 1005-01-029-3428) has been identified as a member of the Browning M2 family through its inclusion in the June 2000 issue of Countermeasure (Vol 21, No 6, available online here). Countermeasure is published by the Army Ground Risk Management Team, and identifies important issues that soldiers should be aware of with regards to risk management and safety. Beyond this connection, there is no specific information on the GAU-10/A, and it is odd that the only online reference would be from a US Army publication as this is a USAF designation.

The XM213/M213 was a modernization and adaptation of existing .50 caliber AN/M2s in inventory for use as a pintle mounted door gun on helicopters.

The GAU-16/A was an improved GAU-15/A with modified grip and sight assemblies for similar applications.

The GAU-18/A, formerly identified as the XM218, is a lightweight variant of the M2/M3, and is used on the USAF's |MH-53J Pavelow II and HH-60 Pave Hawk helicopters. These weapons do not utilize the heavy barrel, and are typically set up as left-hand feed, right-hand charging weapons. In this configuration the gun is fitted with a chute adapter attached to its left hand feed pawl bracket. Thus, the weapon can receive ammunition through a feed chute system connected to internally-mounted ammunition cans. Originally designed to accommodate 1,700 rounds, these cans have since been modified due to space constraints, and now hold about half that amount. However, many aerial gunners find the chute system cumbersome, and opt to install a bracket accommodating the 100-round cans instead (as on the model pictured to the right).

The FN produced M3 series is also in U.S. military service in two versions. One being a fixed remote firing version, the FN M3P, used on the Avenger Air Defense System. The U.S. Army would appear to use this designation for the weapon.

The M3M flexible machine gun has been adopted by the USAF and the USN under the designation GAU-21/A for pintle applications on helicopters.

A long due upgrade program for existing infantry M2s and other M2s currently in U.S. Army service, the E50 finally provides a Quick Change Barrel (QCB) capability, as well as, adding a rail accessory mount, improved flash hider, and a manual safety. While it originally appeared that E50 was within the bounds of the normal U.S. Army designation system, it is actually a developmental project that stands for Enhanced 50 as in enhanced .50 caliber machine gun. The E50 is a conversion kit that can be applied to older weapons — newer machine guns can be produced to this standard, however.